The specification of U.S. Pat. No. 4121103 describes an inspection system based on the absorption of infra-red radiation by water, which is the main constituent of liquid residues, in which light rays including energy in the infra-red region are directed upwards through the bottom of a bottle to be inspected and on to a detection means for detecting the energy of particular infra-red wavelengths which is absorbed if there is any liquid residue in the bottle, thereby to produce a signal to reject any bottle which contains an aqueous residue.
Such a system can operate satisfactorily when inspecting clear bottles, but it cannot satisfactorily inspect coloured bottles which also absorb radiation in or near the infra-red region. Different bottles of the same colour can vary considerably in their absorption of infra-red radiation. The ranges of observed transmission coefficients for different coloured bottles, at the wavelength used for infra-red measurement, are:
for clear bottles--0.5-0.99 PA1 for amber bottles--0.09-0.35 PA1 for green bottles--0.008-0.61
For these reasons a simple measurement of transmitted intensity will not give a reliable indication of water content. The present invention has for an object to alleviate these difficulties and provide an inspection system for coloured and clear bottles and which is capable of use where differently coloured bottles, as well as clear bottles, are inspected on the same line.